144 



MINERAL SALTS ABSORPTION IN PLANTS 



roots have a greater cation exchange capacity than those with fine 

 fibrous roots. On a surface area basis, the C.E.C, of thick roots 

 may be 10-100 times that of thin ones. 



There is no clear relationship between C.E.C. values and the rate 

 at which plants absorb salts, but species with a high C.E.C. value 

 tend to absorb more calcium relative to univalent ions from a given 

 soil. Peas, for example, were found to take up 2-3 times as much 

 calcium as barley plants from a sodium/calcium bentonite clay, but 



Table 15. Cation Exchange Capacity of Roots. 

 (From Drake, Vengris and Colby, 1951). 



only 20-25 per cent as much sodium. This is probably related to the 

 preferential binding of calcium rather than sodium to the exchange 

 sites in the free space of roots, which causes relatively greater 

 amounts of calcium that sodium to be available at the surface of the 

 cytoplasm in roots with a high C.E.C. value than in those with low. 

 The ratio of bivalent to univalent ions adsorbed passively in the cell 

 walls, does not, however, determine the proportions in which these 

 ions are absorbed by the root system as a whole. The ratio of 

 potassium to calcium absorption by barley and oat plants, for 

 example, may be as high as 20 and 60 respectively, indicating that 



